The present invention relates to an adjustable stand having a stand base, a stand column and a supporting arm for optical observation instruments, particularly operation microscopes, wherein the supporting arm is developed as a four-bar linkage of the parallelogram variety.
Operation microscopes must be moved very precisely, both horizontally and vertically. Accordingly, it is necessary to compensate by a counterweight for the weight of the operation microscope carried by the stand. Since space is very limited at the operating table, a compact, self-contained stand construction is particularly desired.
West German Pat. No. 2,161,396 and West German Pat. No. 2,320,266 disclose adjustable stands for optical observation instruments, wherein a counterweight is used to compensate for the weight of the apparatus. For example, for an apparatus weighing 15 kg, the counterweight must also weigh 15 kg, in order to obtain required sensitivity with the observation instrument. Despite optimum compensation for weight, a very heavy mass must, however, be moved by the user, requiring a greater or lesser amount of displacement depending on the speed of displacement. Furthermore, the compensation weight requires considerable free space for its unimpeded movement. Since the equipment of the instrument differs greatly, depending on its purpose of use and the particular requirements of an operation, the compensating-weight requirements necessarily also differ greatly. Precise compensation for instrument weight therefore places an additional demand on the persons using it.
Prospectus M1600-le-III.80 of Firma Wild of Heerbrugg, Germany, discloses a stand which uses a gas spring for weight compensation. This known stand has the disadvantage that an uncompensated residual moment remains, due to friction on the piston and the piston rod, as a result of which the sensitivity of the instrument is impaired. Furthermore, a gas spring is of an open, bulky construction, which is also undesired at the operating table.